CGI.pm has a method called param keywords. It will define 'wsdl' if it is present return an array containing all keywords on QUERY_STRING. I suppose the following code will work:

use CGI;
use List::Util qw(first);

my $q = CGI->new;

if (first { $_ eq 'wsdl' } $q->keywords}) {
    ...    
}
[download]

Update: changed from from $q->param $q->keywords on code.